(Geforce GTX 750) is the best replacement for this graphic card and also the old pc.it consumes lesser power and give more performance and quality, The price of it isn't so expensive though, Its decent...Its price starts from 130$ and i Guarantee u can play most of the games in ultra without lags
Here are comparison between them,If u want (Geforce GTX 750) with a Improved version than its successor is the choice ( GTX 750 TI)
Model - GeForce GTX 750 VS Radeon HD 6750
Manufacturer nVidia AMD
Year February 2014 January 2011
Code Name GM107 Juniper Pro
Fab Process 28 nm 40 nm
Bus PCIe x16 PCIe x16
Memory 1024 MB,(up to 5GB) 512 MB
Core Speed 1020 MHz 725 MHz
Shader Speed N/A MHz (N/A) MHz
Memory Speed 5000 MHz 4000 MHz
Unified Shaders 512 720
Texture Mapping Units 32 36
Render Output Units 16 16
Bus Type GDDR5 GDDR5
Bus Width 128-bit 128-bit
DirectX Version DirectX 11 DirectX 11
OpenGL Version OpenGL 4.4 OpenGL 4.0
Power (Max TDP) 55 watts 86 watts
Shader Model 5.0 5.0
Bandwidth 80000 MB/sec 64000 MB/sec
Texel Rate 32640 Mtexels/sec 26100 Mtexels/sec
Pixel Rate 16320 Mpixels/sec 11600 Mpixels/sec
The price starts from
Geforce GTX 750 - 120$
Geforce GTX 750 Ti - 135$
Enjoy!